Method of making sintered and infiltrated refractory metal electrical contacts



United States Patent 3,505,065 METHOD OF MAKING SINTERED AND IN- FILTRATED REFRACTORY METAL ELEC- TRICAL CONTACTS Childress B. Gwyn, Jr., Wethersfield, Conn., assignor to Talon, Inc., Meadville, Pa., a corporation of Pennsylvania N0 Drawing. Filed Aug. 12, 1968, Ser. No. 751,752 Int. Cl. B22f 7/02 US. Cl. 75208 8 Claims ABSTRACT OF THE DISCLOSURE Method of making refractory metal electrical contacts by sintering and/or infiltration are disclosed wherein a small ercentage of the refractory metal content or a small percentage of copper is provided in the form of an oxide or fluoride which is reduced to the virgin metal during the sintering and/or infiltrating process. Tungsten or molybdenum thus reduced in situ have improved Wetting characteristics, thus facilitating improved infiltration by silver and permitting the production of refractory metal contacts having greater homogeneity than heretofore possible.

BACKGROUND OF THE INVENTION This invention relates to methods for making refractory metal compositions for electrical contacts, and more particularly to the manufacture of such compositions incorporating silver and/ or copper impregnants or infiltrants.

Refractory metal contact compositions are generally constituted of mixtures of refractory metals such as tungsten, molybdenum, or binary mixtures or alloys thereof, having high melting points and resistance to corrosion, with a high conductivity impregnant or infiltrant such as silver, or mixture or alloy thereof. Such compositions are particularly useful for contacts subject to high corrosiontemperature service conditions. In order for the refractory metal contacts to consistently satisfy service conditions, it is necessary that the contacts be uniformly homogenous. Heretofore, such uniform quality has not been attainable.

In the past, refractory metal contact compositions have been prepared by initially blending the refractory metals with binding metals such as copper or silver. Alternatively, the refractory metal has been initially blended with a portion of the binding additive, the composition compacted and sintered, and the balance of the additive subsequently impregnated and/ or infiltrated therein. Each of these methods requires one or more stages of heating to, and beyond, the melting point of the binding metal, generally in an ambient atmosphere of a reducing nature, e.g., in hydrogen or in vacuo. These methods, however, fail to produce consistently uniform refractory metal contacts because of the great difficulty in obtaining uniform wetting and flow of the silver or copper binding metal onto and/ or over the tungsten or molybdenum refractory metal surfaces.

In an effort to obtain the homogeneity not achievable by the above methods, resort has been had to the use of additional additives, such as iron, silicon, copper, nickel, carbon, various fluxes, and the like. The use of these additional additives has failed to produce the required uniformity of result and has also frequently proved to impair the contact make-and-break service characteristics.

Other methods of producing refractory metal contact compositions have included the use of refractory and binding metals of maximum purity in the belief that such would overcome the lack of homogeneity. Also attempted has been the use of tungsten or molybdenum in powder metals plated with nickel, silver or copper prior to further processing. The use of such techniques has, however, failed to produce a repetitive uniform product.

SUMMARY OF THE INVENTION The present invention provides a process for producing refractory metal electrical contacts having very uniform quality by mixing the refractory metal and binding metal with a small percentage by weight of an oxide or fluoride form of the refractory and/ or binding metal so that the oxide or fluoride is reducible to the virgin metal during the sintering or infiltrating processing in suitable ambient temperatures. The reduction of the oxide or fluoride produces the oxygen or fluorine which has been found to cause the binding metal to readily wet and uniformly infiltrate through the entire contact body.

Accordingly, a primary object of this invention is to provide a method for producing homogeneous refractorv metal contacts.

Another object of this invention is to provide a process whereby the uniform wetting and fiow action of the binding metal onto and/or over the refractory metal may be readily accomplished.

Other objects, features and advantages of the instant invention will become apparent from the following description of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the process of the instant invention, a refractory metal such as tungsten or molybdenum is provided in powder form. This refractory metal is blended with a binding or infiltrating metal also in powdered form, such as silver or copper. The blended mixture also includes an additive which is an oxide or fluoride of the refractory and/or binding metal, and is chosen according to the particular refractory and binding metals used. For instance, if the refractory and binding metals are tungsten and silver, respectively, the additive may be tungsten dioxide (WO tungsten trioxide (W0 or tungsten hexafiuoride (WF Where molybdenum is used in place of tungsten preferred additives may be in the form of molybdenum dioxide (M00 molybdenum sesquioxide (M0 0 or molybdenum hexafluoride (MoF While it is possible to use the oxide and fluoride forms of the refractory metals as the additive when the binding metal is silver and/or copper, alternatively it is possible to use copper in several oxide forms. For instance, cuprite (Cu O) or suboxide (Cu O) or cupric (CuO) may be used.

In the process of the instant invention, after the refractory metal, the binding metal and the additive are mixed, they are compacted and then sintered and infiltrated in a reducing atmosphere. Alternatively, the mixtures of the refractory metal, binding metal and additive may be impregnated, sintered and infiltrated in a reducing atmosphere by well known techniques.

In the first process of merely sintering and infiltrating, it is possible to achieve a-maximum weight of 40% of silver or copper in either tungsten or molybdenum. In the process involving impregnation, sintering and infiltrating, 35% silver or copper is achieved after the sintering stage and 6065% silver or copper may be achieved after the infiltration stage.

While each of the above processes has been described, using the metals in their powdered form, it is possible to use metals in other than the powdered form. For instance, copper in the form of shot, slugs, etc. may be used.

The following examples are given to illustrate preferred procedures for carrying out the process of the instant invention, it being intended that the invention should not be construed as restricted to such embodiments.

' 3 Example I A refractory metal contact having a weight percentage composition of 50% molybdenum and 50% silver may be prepared by blending molybdenum in its powdered form suflicient to make up 45% of the weight of the ultimate contact, silver in its powdered form suflicient to 7 make up 50% of the weight and molybdenum dioxide (M suflicient to yield the additional of molybdenum required to provide the 50% molybdenum, 50% silver of the ultimate contact. After the three powders are blended, they are pressed at approximately 20 tons per square inch. The compacted mixture is then sintered in a reducing atmosphere such as hydrogen or dissociated ammonia (NH for a suitable time and temperature cycle, for example, at 2300 F. A micrograph at 500 times magnification showed good homogeneity of a contact produced by this method. A micrograph of 500 times magnification of another contact of 50% molybdenum and 50% silver made under the same conditions as the one described above except without the use of themolybdenum dioxide was also studied. The micrographs showed that the contacts produced in accordance with the instant invention have a superior homogeneity and an almost, if not complete lack of voids, often exhibited by compositions of metals not so prepared.

Example II A contact having a weight percentage composition in its finished form of 75% tungsten and 25% silver may be made by blending tungsten, tungsten trioxide and silver, all in powdered form and approximately 200 mesh. The blend may be made of 25% silver and approximately 65% of the final tungsten weight in pure tungsten powder plus the proper amount of tungsten trioxide (W0 which, when reduced to tungsten, will give an additional of tungsten. After blending, the mixture of the powders is pressed at approximately 20 tons per square inch and then sintered in a reducing atmosphere for a suitable time and temperature cycle, as, for example, twenty minutes at 2000 F. During this cycle, the tungsten trioxide is reduced to pure tungsten in a nascent state and it readily wets with or is coated by the molten silver with which it is in contact.

Example III As another example, a composition may be produced wherein the molybdenum is 60% and the copper is 40% of the weight. In this example, the blended material for sintering or impregnating may be produced of 60% molybdenum content in a pure molybdenum powder form of about 200 mesh and 35% of the final weight of pure copper powder of 100-300 mesh. In addition, a suflicient amount of cuprite (Cu O) included in the mixture to provide the remaining 5% of copper after its reduction to copper during the sintering and/ or infiltrating process is provided. The mixture is compacted at about 30 tons per square inch and heated to approximately 2250 F. for about 30 minutes in a reducing atmosphere.

Example IV A fourth example may be comprised of 50% of a binary alloy of tungsten molybdenum having 70% tungsten, 30% molybdenumand the remainder of the material comprising 10% tungsten and 40% copper. To prepare this contact, the 50% binary alloy may be used in powdered form for convenience and all or part of the 10% tungsten content may be in the form of pure tungsten powder or a tungsten oxide, or alternately, the copper may be partially or entirely pure or in its oxide. The powders are blended together and compacted and placed in suitable containers with a small additional and excess amount of copper in the form of shots, slugs, etc., being placed in contact with at least one part of the blended and com- 4. pactedpowder-mixture. The material is then heated in a reducing atmosphere to a suitable temperature of, for example, 2500 F. for a suflicient period of time of, for example, one hour. The oxides are thereby reduced to the nascent metals and they provide the superior wetting action in accordance with the instant invention.

The reasons for the greater wetting action and infiltration produced by the use of the oxides or fluorides of the refractory or binding metals are not yet fully understood. It is believed that immediately after their reduction to elemental forms, the tungsten, molybdenum and copper are somewhat diflerent in surface area from their subsequent (more aged) state and that in this state they more readily absorb and/0r coat or plate. Such greater wetting action apparently requires traces of oxygen or fluorine and these are momentarily but effectively produced during the reduction of these oxides or fluorides.

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:

1. A method for making refractory metal compositions comprising the steps of:

. (a) combining, in particulate form, a refractory metal, a binding metal and an amount in excess of any impurity and up to about 10% by weight of at least one member of the group consisting of oxides, and fluorides of said refractory metal and of said binding metal; and

(b) heating the aforesaid materials at a temperature above the melting point of said binding metal to reduce said at least one member of said group and wet said refractory metal.

2. The method set forth in claim 1 wherein said at least one member of said group is an oxide of said refractory metal and wherein a portion of said refractory metal which is wet with said binding metal is produced by said reduction of said oxide.

3. The method as set forth in claim 1 wherein said at least one member of said group is an oxide of said binding metal and wherein a portion of said binding metal with which said refractory metal is wet is produced by said reduction of said oxide.

4. The method as set forth in claim 1 wherein said refractory metal, said binding metal and said at least one member of said group are combined in powdered form and are blended together and compacted in a mixture and wherein said compacted mixture is sintered and infiltrated in a reducing atmosphere to cause said wetting.

References Cited UNITED STATES PATENTS 2,998,641 9/1961 Atkinson 75203 XR 3,050,386 8/1962 Von Dohren 75222 XR 3,373,018 3/1968 Oxley 75224 XR 3,407,048 10/1968 Krock 75208 XR 3,438,753 4/1969 Kenney 75-208 XR 3,441,409 4/ 1969 Burr 75-224 XR BENJAMIN R. PADGETT, Primary Examiner ARTHUR J. STEINER, Assistant Examiner US. Cl. X.R. 

